This invention relates to a capillary electrophoresis system and a method of controlling the gas pressure inside a sample tube thereof as a sample is injected therefrom into a capillary tube.
Methods of injecting a sample into a capillary tube include the so-called syphon method, potential difference method and compression method. Of the above, the compression method is one whereby the sample is preliminarily placed inside a sample tube and the gas pressure therein is increased by introducing high-pressure nitrogen gas or the like thereinto. If such a gas is made to pass through a solenoid valve on its way into the sample tube, the gas pressure inside the sample tube can be controlled by opening and closing this valve. In order to accurately inject a specified amount of the sample from the sample tube into the capillary tube in this manner, it is necessary to keep the gas pressure inside the sample tube at a specified constant level.
One method of controlling the gas pressure for such a purpose is to preliminarily set an upper limit value (such as 51 gf/cm.sup.2) and a lower limit value (such as 49 gf/cm.sup.2) for a given target pressure level (such as 50 gf/cm.sup.2) and to close the valve if the pressure inside the sample tube exceeds the upper limit value, while opening the valve if the pressure is below the lower limit value. Because of the finite volume of the gas flow system, however, there is a time lag between the opening or closing of the valve and the resultant change in the gas pressure. The gas pressure may even keep oscillating beyond and over the specified upper and lower limit values. Since the pressure inside the sample tube is generally low (much lower than 100 gf/cm.sup.2), furthermore, it is easily affected by changes in the ambient temperature or even a small leak in the gas flow system. As a result, the gas pressure may change significantly and the variations of its maxima and minima may grow larger. In such a situation, it is not possible to set the gas pressure accurately to a specified level. Although this time lag between the opening and closing of the valve and the pressure change can be reduced by making the total volume of the gas flow system smaller, the changes in pressure will become too large making it impossible to keep the pressure constant inside the sample tube.
It is therefore an object of the present invention to provide a capillary electrophoresis system and a method by which gas pressure can be accurately controlled.